The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to the detection of a substrate in a carrier head.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar.
Chemical mechanical polishing (CMP) is one accepted method of planarizing a substrate surface. This planarization method typically requires that the substrate be mounted to a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad or moving polishing belt. The polishing pad may be a xe2x80x9cstandardxe2x80x9d pad with a durable roughened surface, or a fixed-abrasive pad with abrasive particles embedded in a binder. The carrier provides a controllable load on the substrate to press it against the polishing pad. In addition, the carrier may rotate to affect the relative velocity distribution over the surface of the substrate. A polishing slurry, including at least one chemically-reactive agent, and an abrasive if a standard pad is being used, may be distributed over the polishing pad.
Typically, the carrier head is used to remove the substrate from the polishing pad after the polishing process has been completed. The substrate is vacuum-chucked to the underside of the carrier head. When the carrier head is retracted, the substrate is lifted off the polishing pad.
One problem that has been encountered in CMP is that the substrate may not be lifted by the carrier head. For example, if the surface tension binding the substrate to the polishing pad is greater than the force binding the substrate to the carrier head, then the substrate will remain on the polishing pad when the carrier head retracts. Also, if a defective substrate fractures during polishing, then the carrier head may be unable to remove the fractured substrate from the polishing pad.
A related problem is that the attachment of the substrate to the carrier head may fail, and the substrate may detach from the carrier head. This may occur if, for example, the substrate was attached to the carrier head by surface tension alone, rather than in combination with vacuum-chucking.
As such, an operator may not know that the carrier head no longer carries the substrate. The CMP apparatus will continue to operate even though the substrate is no longer present in the carrier head. This may decrease throughput. In addition, a loose substrate, i.e., one not attached to a carrier head, may be knocked about by the moving components of the CMP apparatus, potentially damaging the substrate or the polishing pad, or leaving debris which may damage other substrates.
Another problem encountered in CMP is the difficulty of determining whether the substrate is present in the carrier head. Because the substrate is located beneath the carrier head, it is difficult to determine by visual inspection whether the substrate is present in and properly attached to the carrier head. In addition, optical detection techniques are impeded by the presence of slurry.
A carrier head may include a rigid base having a bottom surface which serves as a substrate receiving surface. Multiple channels extend through the base to the substrate receiving surface. A pump or vacuum source can apply a vacuum to the channels. When air is pumped out of the channels, the substrate will be vacuum-chucked to the bottom surface of the base. A pressure sensor may be connected to a pressure line between the vacuum source and the channels in the carrier head. If the substrate was not successfully vacuum-chucked to the carrier head, then the channels will be open and air or other fluid will leak into the channels. On the other hand, if the substrate was successfully vacuum-chucked to the carrier head, then the channels will be sealed and air will not leak into the channels. Consequently, the pressure sensor will measure a higher vacuum or lower pressure when the substrate is successfully vacuum-chucked to the underside of the carrier head as compared to when the substrate is not attached to the carrier head.
Unfortunately, there are several problems with this method of detecting the presence of a substrate in the carrier head. Corrosive slurry may be suctioned into the channels and contaminate the carrier head. In addition, the threshold pressure for determining whether the substrate has been lifted from the polishing pad must be determined experimentally.
Accordingly, it would be useful to provide a CMP system capable of reliably sensing the presence of a substrate in a carrier head. It would also be useful if such a system could operate without exposing the interior of the carrier head to contamination by a slurry.
In one aspect, the invention is directed to a carrier head that has a base, a flexible member that defines a first chamber and has a lower face that provides a substrate receiving surface, and a valve in the carrier head that forms part of a substrate detection system. The valve includes a valve stem that contacts an upper surface of the flexible membrane so that if a substrate is attached to the lower surface of the flexible membrane when the first chamber is evacuated, the valve is actuated to generate a signal to the substrate detection system.
Implementations of the invention may include the following features. The valve may be positioned in a passage that fluidly couples the first chamber to a second chamber. The valve may be biased in an open or closed position, and actuation of the valve may close or open the valve. The valve stem may extend through an aperture in a support structure, and may project slightly beyond a lower surface of the support structure. The support structure may be movable relative to the base. The valve may be biased by a spring, and the spring constant of the spring may be selected so that the force from the spring is sufficient to counteract a force from a flexible membrane when the substrate is not attached, but is insufficient to counteract a force from a flexible membrane when the substrate is attached. The valve stem may contacts the upper surface of the flexible membrane if the first chamber is evacuated. The flexible membrane may wrap around a lower portion of the valve if the substrate is not present.
In another implementation, the carrier head has a base, a flexible member that defines a first chamber and has a lower face that provides a substrate receiving surface, and a valve in the carrier head that forms part of a substrate detection system. The valve includes a valve stem that projects past a support surface, so that if the first chamber is evacuated and a substrate is attached to the lower surface of the flexible membrane, the substrate abuts the support surface and actuates the valve.
In another implementation, the carrier head has a base, a flexible member that defines a first chamber and has a lower face that provides a substrate receiving surface, and a plurality of valves in the carrier head that form part of a wafer detection system. If a substrate is attached to the flexible membrane when the first chamber is evacuated, either of the valves may be actuated to generate a signal to the wafer detection system.
In another implementation, the carrier head has a base, a flexible member that defines a first chamber and has a lower face that provides a substrate receiving surface, and a plurality of valves in the carrier head that form part of a wafer detection system. If a substrate is attached to the flexible membrane when the first chamber is evacuated, both of the valves must be actuated to generate a signal to the wafer detection system.
In another implementation, the carrier head has a base, a flexible member that defines a first chamber and has a lower face that provides a substrate receiving surface, a second chamber, a passage through the base between the first and second chambers, a first valve that is biased open and actuates to close the passage if the first chamber is evacuated a substrate is attached to the flexible membrane when the first chamber, and a second valve connected in series with the first valve, the second valve biased closed and actuatable to open the passage if the second chamber is evacuated.
Advantages of the invention include the following. The CMP apparatus includes a sensor to detect whether the substrate is properly attached to the carrier head. The sensor is less prone to false alarms.
Other advantages and features of the invention become apparent from the following description, including the drawings and claims.